ECOLOGY OF PLANTS
Introduction.—The word ecology is derived from oTKOS, a house (habitat), and Xiyos, a discourse. As a botanical term, ecology denotes that branch of botany which comprises the study of the relations of the individual plant, or the species, or the plant community with the habitat. Following SchrSter1 (Flahault and Schroter, 1910 : 24), the term autecology may be used for the study of the habitat conditions in relation to the single species, and the term synecology for this study in relation to plant communities.
From the phytogeographical standpoint, ecology is frequently termed ecological plant geography. Thus Warming2 (1901:1 and 2)
Flahault and Schroter, Phytogeographical Nomenclature: reports and propositions (Zurich, 1910).
2 Warming, Oecology of Plants (Oxford, 1909).
subdivided plant geography into floristic plant geography and ecological plant geography. The former is concerned with the division of the earth's surface into major districts characterized by particular plants or taxonomic groups of plants, with the subdivision of these floristic districts, and with the geographical distribution (both past and present) of the various taxonomic units, such as species, genera, and families. On the other hand, ecological plant geography seeks to 'ascertain the distribution of plant communities, such as associations and formations, and enquires into the nature of the factors of the habitat which are related to the distribution of plants—plant forms, species, and communities. In a general way, floristic plant geography is concerned with species, ecological plant geography with vegetation. The study of the distribution of species dates back to the time of the early systematists, the study of vegetation to the time of the early botanical travellers. Humboldt,' for example, defined his view of the scope of plant geography as follows: " C'est cette science qui considere les vegetaux sous les rapports de leur association locale clans les differents climats " (1807: i4).
The Habitat.—The term habitat, in its widest sense, includes all the factors of the environment which affect a plant or a plant community, though the term is frequently used to signify only some of these factors. The factors of the habitat may be grouped as follows: geographical, physical, and biological.
Geographical Factors.—Geographical position determines the particular species of plants which grow in any particular locality. This matter is bound up with the centres of origin and with the past migrations of species; and such questions are usually treated as a part of floristic plant geography. Here, therefore, floristics and ecology meet. Flahault and Schroter,2 in defining the term habitat, appear to exclude all geographical factors. They state that " the term habitat is understood to include everything relating to the factors operative in a geographically defined locality, so far as these factors influence plants " (1910: 24) ; but the exclusion of geographical and historical factors from the concept of the habitat does not appear to be either desirable or logical.
Physical Factors.—These are frequently classified as edaphic or soil factors and climatic factors; but there is no sharp line of demarcation between them. Edaphic factors include all those relating to the soil. The water content of the soil, its mineral content, its humus content, its temperature, and its physical characteristics, such as its depth and the size of its component particles are all edaphic factors. Climatic factors include all those relating to atmospheric temperature, rainfall, atmospheric humidity, and light and shade. Factors connected with altitude, aspect, and exposure to winds are also climatic: such are often spoken of as physiographical factors. The difficulty of sharply delimiting edaphic and climatic factors is seen in the case of temperature. Soil temperature is partly dependent on the direct rays of the sun, partly on the colour and constitution of the soil, and partly on the water content of the soil. Again, the temperature of the air is affected by radiation from the soil; and radiation differs in various soils.
Biological Factors.—These include the reactions of plants and animals on the habitat. Here again, no sharp boundary-line can be drawn. In one sense, the accumulation of humus and peat is a biological factor, as it is related to the work of organisms in the soil; but the occurrence or otherwise of these organisms in the soil is probably related to definite edaphic and climatic conditions. Again, the well-known action of earthworms may be said to be a biological work; but the resulting aeration of the soil causes edaphic differences; and earthworms are absent from certain soils, such as peat. The pollination of flowers and the dispersal of seeds by various animals are biological factors; but pollination and dispersal by the wind cannot be so regarded. The influence of man on plants and vegetation is also a biological factor, which is frequently ignored as such, and treated as if it were a thing apart.
When the nature and effect of ecological factors have become more fully understood, it will be possible to dispense with the above artificial classification of factors, and to frame one depending on the action of the various factors; but such a classification is not possible in the present state of knowledge.
Ecology and Physiology.—Whilst our knowledge of the nature and effect of habitat is still in a very rudimentary condition, much progress has been made in recent years in the study of plant communities; but even here the questions involved in relating the facts of the distribution of plant communities to the
' Humboldt and Bonpland, Essai sur la geographie des plantes (Paris, 1807).
2 Flahault and Schroter (op. cit.).factors of the habitat are very imperfectly understood. This is due to a lack of precise knowledge of the various habitat factors and also of the responses made by plants to these factors. Until much more advance has been made by ecologists in the investigation of the nature of habitat factors, and until the effect of the factors on the plants has been more closely investigated by physiologists, it will remain impossible to place ecology on a physiological basis: all that is possible at present is to give a physiological bias to certain aspects of ecological research. Obviously no more than this is possible until physiologists are able to state much more precisely than at present what is the influence of common salt on the plants of salt-marshes, of the action of calcium carbonate on plants of calcareous soils, and of the action of humous compounds on plants of fens and peat moors.
Ecological Classes.—Many attempts have been made to divide plants and plant communities into classes depending on habitat factors. One of the best known classifications on these lines is that by Warming.' Warming recognized and defined four ecological classes as follows:
Hydrophytes.—These live in a watery or wet substratum, with at least 8o% of water. Warming included plants of peat-bogs among his hydrophytes.
Xerophytes.—These are plants which live in very dry places, where the substratum has less than io% of water.
Halophytes.—These are plants living in situations where the sub-stratum contains a high proportion of sodium chloride.
Mesophytes.--These are plants which live in localities which are neither specially dry nor specially wet nor specially salty.
Such terms as hydrophytes, xerophytes, and halophytes had been used by plant geographers before Warming's time e.g., by Schouw;4 and the terms evidently supply a want felt by botanists as they have come into general use. However, the terms are incapable of exact definition, and are only useful when used in a very general way. The above classification by Warming, although it was without doubt the best ecological classification which had, at the time, been put forward, has not escaped criticism. The criticisms were directed chiefly to the inclusion of sand dune plants among halophytes, to the exclusion of halophytes from xerophytes, to the inclusion of " bog xerophytes " among hydrophytes, to the inclusion of all conifers among xerophytes and of all deciduous trees among mesophytes, and to the group of mesophytes in general.
Schimper s made a distinct advance when he distinguished between physical and physiological dryness or wetness of the soil. A soil may be physically wet; but if the plants absorb the water only with difficulty, as in a salt marsh, then the soil is, as regards plants, physiologically dry. All soils which are physically dry are also physiologically dry; and hence only the physiological dryness or wetness of soils need be considered in ecology.
Schimper used the term xerophytes to include plants which live in soils which are physiologically dry, and the term hygrophytes those which live in soils which are physiologically wet or damp. Schimper recognized that the two classes are connected by transitional forms, and that it is useless to attempt to give the matter a statistical basis. It is only in a general sense like Schimper's that such ecological terms as xerophytes have any value; and it is not possible, at least at present, to frame ecological classes, which shall have a high scientific value, on a basis of this nature. Whilst Schimper objected to the constitution of a special category, such as mesophytes, to include all plants which are neither pronounced xerophytes nor pronounced hygrophytes, he recognized the necessity of a third class in which to place those
3 Warming, Planlesamfund, Kjobenhavn, I895; (See German trans. by Knoblauch, " Lehrbuch der okologischen Pflanzengeographie " (Berlin, 1896) ; new German ed. by Graebner (Berlin, 1902).
' Schouw, Grundtraek til en almindelig Plantegeografie (Kjobenhavn, 1822) ; German trans., " Grundzuge ewer allegemeinen Pflanzengeographie " (Berlin, 1823).
Schimper, Pflanzengeographie auf physiologischer Grundlage (Berlin, 1898) ; Eng. trans. by Fisher, " Plant Geography upon a Physiological Basis " (Oxford, 1903-1904).
ECOLOGY]
plants which, like deciduous trees and bulbous plants, are hygrophytes during one season of the year and xerophytes during another season of the year. Such plants, which comprise the great majority of the species of the central European flora, Schimper termed tropophytes.
Recently, Warming' (1909 136), assisted by Vahl, has modified his earlier classification, and adopted the following:
A. The soil (in the widest sense) is very wet, and the abundant water is available to the plant (at least in hydrophytes).
1. Hydrophytes.—These include plants of the plankton, or microphytes that float free on water, of the pleuslon, or macrophytes which float on or are suspended in water, and of the benthos, or all aquatic plants which are fixed to the substratum.
2. Helophytes.—These are marsh plants which normally have their roots in soaking soil but whose branches and foliage are more or less aerial. Warming admits there is no sharp limit between marsh plants and land plants; and it seems equally obvious that there is no sharp limit between some of his helophytes and some of his hydrophytes. For example, the difference between aquatic plants with floating leaves, such as the yellow water-lily (Nymphaea lutea) and those with erect leaves, such as Typha angustifolia, is probably more apparent than real. Among helophytes, Warming places plants of the reed swamp, and includes such trees as the alder (Alnus rotundifolia), willows (e.g., Salix alba, S. fragilis, S. cinerea, S. pentandra), birch, and pine, when these grow in marshy places.
B. The soil is physiologically dry.
3. Oxylophytes.—These plants, sometimes spoken of as " bog xerophytes," grow in soils which contain an abundance of free humous compounds, and include plants which grow on fens and moors.
4. Psychrophytes.—These include the plants which grow on the cold soils of subniveal and polar districts.
5. Halophytes.—These are plants which grow on saline soils.
C. The soil is physically dry.
6. Lithophytes.—These are plants which grow on " true rock," but not " on the loose soil covering rock, even though this may entertain species that are very intimately associated with the rock. Still to this limitation an exception must be made in favour of the vegetation growing in clefts and niches " (Warming, 1909 : 240). Many Algae, lichens, and mosses are included among lithophytes, and also Saxifraga Aizoon, S. oppositifolia, Silene acaulis, and Gnaphalium luteo-album.
7. Psammophytes.—These are plants which grow on sand and ggravel. Plants of sand-dunes, whether in maritime or inland localities, are psammophytes, as well as plants (such as Cd7luna vulgaris) of dune heaths, dune "bushland " or scrub, and dune forest.
8. Chersophytes.—Here are placed certain " xerophytic perennial herbs " which occur on " particular dry kinds of soil, such as lime-stone rocks, stiff clay, and so forth " (Warming, 1909 : 289). '
D. The climate is very dry, and the properties of the soil are decided by climate.
9. Eremophytes.—Under this term, are placed plants of deserts and steppes.
to. Psilophytes.—Here are placed plants found in " savannah-vegetation," viz. (i.) " thorny savannah-vegetation, including: (a) orchard-scrub, (b) thorn-bushland and thorn-forest; (ii.) true savannah tropical and sub-tropical savannah ; (iii.) savannah-forest, including bush-forest in Africa and ' campos serrados ' in Brazil
(Warming, 1909 : 293 et seq.).
11. Sclerophyllous formations, e.g., garigues, maquis, and forests of evergreen oaks (Q. Ilex, Q. Ballota, Q. Suber), and.of Eucalyptus spp.
E. The soil is physically or physiologically dry.
12. Coniferous forest formations, e.g., of Pinus sylvestris, Picea excelsa, Abies pectinata, Larix sibirica, L. decidua.
F. " Soil and climate favour the development of mesophilous formations."
13. Mesophytes.—Warming defines mesophytes as " plants that show a preference for soil and air of moderate humidity, and avoid soil with standing water or containing a great abundance of salts " (1909 : 317). Under mesophytes, Warming places plants occurring in " Arctic and Alpine mat-grassland and mat-herbage," in " mat-vegetation of the Alps," in meadows, in pasture on cultivated soil, in " mesophytic bushland," in deciduous dicotyledonous forests, and in evergreen dicotyledonous forests.
This. new system of Warming's, whilst probably too involved ever to come into general use, must be taken as superseding his older one;2 and perhaps the best course open to botanists is to select such terms as appear to be helpful, and to use the selected terms in a general kind of way and without demanding any precise definitions of them: it must also be borne in mind that the
'' Warming (1909,"op. ciL). 2 Ibid, (1894, op. cit.).
76.1
various classes are neither mutually exclusive nor of equivalent rank. From this point of view, the following terms will perhaps be found the most serviceable:
Hydrophytes (submerged aquatic plants).—Plants whose vegetative organs live wholly in water; e.g., most Algae, many mosses, such as Fontinalis spp., and liverworts, such as Jungermannia spp.; a few Pteridophytes, such as Pilularia spp., Isoetes spp. ; several flowering plants, such as Potamogeton pectinatus, Ceratophyllum spp., Hottonia palustris, Utricularia spp., Littorella lacustris.
Hemi-hydrophytes (swamp plants, marsh plants, &c.).—Plants whose vegetative organs are partly submerged and partly aerial; Vaucheria terrestris, Philonotis Fontana, Seapania undulata, Marsilia spp., Salvinia ratans, Azolla spp., Equisetum limosum, Typha angustifolia, Phragmites communis, Scirpus lacustris, Nymphaea lutea, Oenanthe fistulosa, Bidens cernua.
Hygrophytes.—Plants which are sub-evergreen or evergreen but not sclerophyllous, and which live in moist soils; e.g., Lastraea Filix-mas, Poe. pratensis, Carex ovalis, Plantago lanceolata, and Achillaea Millefolium.
Xerophytes.—Plants which grow in very dry soils; e.g., most lichens, Ammophila (Psamma) arenaria, Elymus arenarius, Ana-basis aretioides, Zilla macroptera, Sedum acre, Bupleurum spinosum, Artemisia herba-alba, Zollikofferia arborescens.
Halophytes.—Plants which grow in very saline soils; e.g., Triglochin maritimum, Salicornia spp., Zygophyllum cornutum, Aster Tripolium, Artemisia maritima. It should be recognized, however, that " a halophyte, in fact, is one form of xerophyte " (Warming, 1909: 219).
Sclerophyllous Plants.—These are plants with evergreen leathery leaves, and typical of tropical, sub-tropical, and warm temperate regions; e.g., Quercus Suber, Ilex Aquifolium, Hedera Helix, Eucalyptus Globulus, Rosmarinus officinalis. Sclerophyllous leaves are usually characterized by entire or sub-entire margins, a thick cuticle, small but rarely sunken stomata, a well-developed and close-set palisade tissue and a feeble system of air-spaces.
Hydro-xerophytes (" bog xerophytes ").—Plants which live in wet, peaty soils, and which possess aeration channels and xenophilous leaves; e.g., Cladium Mariscus, Eriophorum angustifolium, Rubus Chamaemorus, and Vaccinium Vitis-Idaea. The term " oxylophyte " is open to the objection that some peaty waters are alkaline, and not acidic as the term implies. Many plants of peaty soils are sclerophyllous.
Tropophytes.—Plants which are hygrophytes during some favour-able part of the year and xerophytes during the rest of the year; e.g., deciduous trees and shrubs, deciduous herbaceous plants with underground perennating organs, and annuals and ephemerals.
Plant Communities:—The study of plant communities (Formationslehre or synecology) has made much progress in recent years. Even here, however, general agreement has not been reached; and the questions involved in relating the facts of the distribution of plant communities to the factors of the habitat are very imperfectly understood. Plant communities may be classified as follows:
A plant association is a community of definite floristic composition: it may be characterized by a single dominant species; or, on the other hand, it may be characterized by a number of prominent species, one of which is abundant here, another there, whilst elsewhere two or more species may share dominance. The former are pure associations, and are well illustrated by a heather moor, where Calluna vulgaris is the dominant plant. The latter are mixed associations, such as fens, where different facies are produced by the varying abundance of characteristic plants, such as Cladium Mariscus, Phragmites communis, Molina coerulea, Calamagrostis lanceolata, and Juncus obtusiflorus. The different facies are possibly related to slight differences in a generally uniform habitat: it is unscientific to regard them as due to chance; still, in the majority of cases, , the causes of the different facies have not been demonstrated. A local aggregation of a species other than the dominant' one in an association brings about a plant society; for example, societies of Erica Tetralix, of Scirpus caespitosus, of Molinia coerulea, of Carex curta, of Narthecium ossifragum, and others may occur within an association of Calluna vulgaris. The plant societies are also doubtless due to slight variations of the habitat.
The plant association is sometimes referred to in technical language;3 the termination -diem is added to the stem of the generic name, and the specific name is put in the genitive. Thus
' See Moss, " The Fundamental Units of Vegetation: historical development of the concepts 'of the plant association and the plant formation." Botany School (Cambridge, 3010).
an association of Quercus sessiliflara may be referred to as a Quercetum sessiliflorae.
A plant formation is a group of associations occupying habitats which are in essentials identical with each other. Thus, associations of Agropyrum (Triticum) junceum, of Carex arenaria, of Ammophila (Psamma) arenaria, and of other plants occur on sand dunes: the associations are related by the general identity of the habitat conditions, namely, the physiological dryness and the loose soil; but they are separated by differences in floristic composition, especially by different dominant species, and by minor differences of the common habitat. The whole set of associations on the sand dunes constitutes a plant formation.
The plant formation may be designated in technical language by the termination -ion added to a stem denoting the habitat. Thus, a sand dune formation may be termed an Arenarion. The associational term, in the genitive, may be added to the formational term to indicate the relationship of the formation and the association; thus, a plant association of Ammophila arenaria belonging to the plant formation of the sand dunes may be designated an Arenarion Ammophilae-arenariae (cf. Moss, op. cit. 1910: 43).
The question of universal names for vegetation units is bound up with that of the universality or otherwise of particular formations. " Remote regions which are floristically distinct
. may possess areas physically almost identical and yet be covered by different formations" (Clements,' 1905 : 203). For example, the sand dunes of North America and those of western Europe are widely separated in geographical position and there-fore in floristic composition, yet they are related by common physical factors. This relationship may be indicated by the addition of some prefix to the formational name. For example, an Arenarion in one climatic or geographical region might be termed an a-Arenarion and one in a different region a i3-Arenarion, and so on (Moss, loc. cit.).
It is, however, frequently desirable to consider such allied formations as a single group. Such a group of formations may be designated a plant federation: and this term may be defined as a group of formations, which are characterized by common edaphic factors of the habitat, and which occur in any geographical region. Thus, different geographical or climatic regions are characterized, by salt marshes. The latter all agree in their edaphic characteristics; but they differ climatically and in floristic composition. The salt marshes of a given region constitute a single plant formation : the salt marsh formations of the whole world constitute a plant federation.
Again, it is possible to arrange plant associations into groups related by a common plant form. Thus woodland associations may be classified as deciduous forests, coniferous forests, sclerophyllous forests, &c. These, in a general way, are the "formations " of Warming,2 and (in part) the " climatic formations " of Schimper .2 Thus the various reed-swamps of the whole world constitute a " formation " in Warming's sense (1909: 187).
There is much difference of opinion among ecologists and plant geographers as to which of these points of view is the most fundamental. Among British authorities, it is now customary to adopt the position of Clements, who states (1905:292) that " the connexion between formation and habitat is so close that any application of the term to a division greater or smaller than the habitat is both illogical and unfortunate," and that (1905:18) " habitats are inseparable from the formations which they bear " (cf. Moss, 191o).
From the standpoint of plant communities, it is convenient to divide the earth's surface into (i) tropical districts;4 (2) sub-
F. E. Clements, Research Methods in Ecology (1905), Lincoln, Neb., U.S.A.
2 Warming (1909, op. Cit.). 6 Schimper (1898, op. cit.).
4 The nomenclature of the terms (floristic as well as ecological) used in geographical botany is in a very confused state. In the present article, the term " district " is used in a general sense to indicate any definite portion of the earth's surface. For a discussion of such phytogeographical terms, see Flahault, " Premier essai de nomenclature phytogbographique," in Bull. Soc. languedocienne de Geogr. (19o1); and also in Bull. Torr. Bet. Club (1901).tropical and warm temperate districts; (3) temperate districts; (4) cold temperate and frigid districts.
i. Tropical Districts.—The vegetation of tropical districts has been subdivided by Schimper (1903:260, et seq.) as follows:—O.) Tropical woodland: (a) rain forest, (b) monsoon forest, (c) savana forest, (d) thorn forest. (ii.) Tropical grassland: (a) savana, (b) steppe. (iii.) Tropical desert: (a) scrub, (b) succulent plants, (c) perennial herbs.
Schimper regards the minor divisions as groups of " climatic formations "; and he also distinguishes certain tropical " edaphic formations," such as mangrove swamps. He states that rain forests and high monsoon forests in the tropics occur when the average rainfall is over 70 in. (178 cm.) per annum, and that tropical thorn forest may prevail when the mean annual rainfall is below 35 in.
A tropical rain forest exhibits great variety both of species of plant and of plant forms. There is great diversity in the trees and masses of tangled lianes, and a wealth of flowers in the leafy forest crown. Humboldt 6 points out that whilst temperate forests frequently furnish pure associations, such uniformity of association is usually absent from the tropics. Some tropical forests exhibit dense foliage from the forest floor to the topmost leafy layer; and the traveller finds the mass of foliage almost impenetrable. Other tropical forests afford a free passage and a clear outlook. It is obvious that tropical forests will eventually be subdivided into plant associations; but the difficulties of deter-mining the relative abundance of the species of plants in the upper layers of tropical rain and monsoon forests are very great. One of the best known results of the great struggle for light which takes place in tropical forests is the number of epiphytic plants which grow on the high branches of the trees.
The leaves of the trees are frequently of leathery consistency, very glossy, usually evergreen, entire or nearly so, and seldom hairy ; and thus they agree closely with the leaves of sclerophyllous forest generally.
Monsoon forests are characteristic of localities with a seasonal rainfall. The trees usually lose their foliage during the dry season and renew it during the monsoon rains. With a less abundant rainfall, savana forest and thorn forest occur. Less precipitation induces tropical grassland, which, according to Schimper (1903: 346) is of the savana type; but Warming (1909:327) thinks that all grassland in the tropics is artificial. Still greater drought induces desert vegetation; but, as deserts are more characteristic of subtropical districts, they are discussed later on.
Mangrove swamps, or tropical tidal forests, occur in saline or brackish swamps on flat, muddy shores in the tropics; and, being almost independent of atmospheric precipitations, Schimper regards them as " edaphic formations." However, they are climatic communities in the sense that they occur only in hot districts. Cases such as this illustrate the difficulty of regarding the distinction between " climatic formations " and " edaphic formations " as absolute. The plants exhibit markedly xerophilous structures; and many of the fruits and seeds of the mangrove trees and shrubs are provided with devices to enable them to float and with curious pneumatophores or " prop roots." The latter serve as supports and also as a means of supplying air to the parts buried in the mud. The seedlings of characteristic species of Rhizophoraceae germinate on the trees, and probably perform some assimilatory work by means of the hypocotyl.
Other tropical " edaphic formations " occur on sandy shores, where the, creeping Ipomoea biloba (Pes-caprae) and trees of Barringtonia form characteristic plant associations.
The succession of associations on new soils of a tropical shore has recently been described by Ernst .4
2. Warm Temperate and Subtropical Districts.—In subtropical and warm temperate districts, characterized by mild and rainy winters and hot and dry summers, we find two types of forests. First, there are forests of evergreen trees, with thick, -leathery leaves. Such forests are known as sclerophyllous forests, and they occur in the Mediterranean region, in south-west Africa, in south and south-west Australia, in central Chile, and in western California. In the Mediterranean district, forests of this type are sometimes dominated by the Cork Oak (Quercus Saber), sometimes by the Holm Oak (Q. Ilex). When these forests become degenerate, maquis and garigues respectively are produced. Maquis and garigues are characterized by the abundance of shrubs and under-shrubs, especially by shrubby Leguminous plants, and by species Cistus and Lavandula. Secondly, there are forests of coniferous trees. In the Mediterranean region, even at comparatively low altitudes, forests occur of the maritime pine (Pines marztima) and of the Aleppo pine (P. halepensis); and these forests are also related to maquis and garigues respectively in the same way as the evergreen oaks. The occurrence of forests of this type in the Mediterranean and in Arctic regions, whose dominant species belong to the same genus (Pinus) and to the same plant form, renders it
6 Humboldt, Eng. trans. by Sabine, Aspects of Nature (London, 1849)
6 Eng. trans. by Seward, The New Flora of the Yolaa4iic Island of Krakatau (Cambridge, 1908).
difficult to regard " coniferous forests " as a natural ecological group. At much higher altitudes, in the south-west of the Mediterranean region, forests occur of the Atlantic cedar (Cedrus atlantica). These occur from about 4000 ft. (1219 m.) to about 7000 ft. (2133 m.) on the Atlas Mountains. Some sclerophyllous forests of the eastern Atlas Mountains are, owing to a comparatively high rainfall, characterized by many deciduous trees, such as Fraxinus oxyphylla, Ulmus campestris (auct. alg.), Alnus rotundifolia, Salix pedicellata, Prunus avium, &c.; and thus they have some elements in common with the deciduous forests of central Europe.
The forests of these subtropical and warm temperate regions are situated near the sea or in mountainous regions, and (as already stated) are characterized by winter rains. In inland localities, where the rainfall is much lower, steppes occur. For example, in southern Algeria, a region of steppes is situated on a flat plateau, about 3000 ft. (914 metres) high, between the southern slopes of the Tell Atlas and the northern slopes of the Saharan Atlas. The rainfall, which occurs chiefly in winter, only averages about 10 in. (254 mm.) per annum. Here we find open plant associations of Haifa or Esparto Grass (Stipa tenacissima) alternating with steppes of Chih (Artemisia herby-alba) ; and each plant association extends for several scores of miles. In the hollows of this steppe region, salt water lakes occur, known as Chotts; and on the saline soils surrounding the Chotts, a salt marsh formation occurs, with species of Salicornia, some of which are undershrubs.
Where the rainfall is still lower, deserts occur. At Ghardaia, in south-eastern Algeria, the mean annual rainfall, from 1887 to 1892, was about 4a in. (114 mm.). In 189o, it fell as low as 2 in. (53 mm.) (Schimper, 1903 : 6o6). At Beni Ounif and Colomb Bechar, in south-western Algeria, I was informed, in March 1910, that there had been no rain for about three years. Here the gravelly desert is characterized by " cushion plants," such as Anabasis aretioides ; by " switch plants," such as Retama Retam ; and specially by spiny plants, such as Zizyphus Lotus and Zilla macropteris; whereas succulent plants are rare. Both in the steppe and in the desert, small ephemeral species occur on the bare ground away from the large plants and especially in the wadis. Steppe and desert formations are of the open type.
3. Temperate Districts.—Temperate districts are characterized by forests of deciduous trees and of coniferous trees, the latter being of different species from those of the warm temperate districts, but frequently of the same plant form. The identity of plant form of many of the conifers of both temperate and of warm temperate districts is probably a matter of phylogenetic and not of ecological importance.
Britain is fairly typical of the west European district. In these islands, we find forests 1 or woods of oak (Quercus Robur and sessiliflora), of birch (Betula tomentosa), of ash (Fraxinus excelsior), and of beech (Fagus sylvatica). In central Scotland, forests occur of Pinus sylvestris; and, in south-eastern England, extensive plantations and self-sown woods occur of the same species.
Just as in the Mediterranean region, the degeneration of forests has given rise to maquis and garigues, so in western Europe, the degeneration of forests has brought about different types of grass-land, heaths, and moors.
4. Cold Temperate and Frigid Districts.—In the coldest portion of the north temperate zone, forests of dwarfed trees occur, and these occasionally spread into the Arctic region itself (Schimper, 1904: 685). Schimper distinguishes moss tundra, Potytrichum tundra, and lichen tundra; and the lichen tundra is subdivided into Cladonia tundra, Platysma tundra, and Alectoria heath. Where the climate is most rigorous, rock tundra occurs (p. 685).
The types of vegetation (tropical forests, sclerophyllous forest, temperate forests, tundra, &c.) thus briefly outlined are groups of Schimper's " climatic formations." Such groups are interesting in that they are vegetation units whose physiognomy is, in a broad sense, related more to climatic than to edaphic conditions. For example, Schimper, after describing the sclerophyllous woodland of the Mediterranean district and of the Cape district, says: " The scrub of West and South Australia in its ecological aspect resembles so completely the other sclerophyllous formations that a description of it must seem a repetition." This resemblance, however, only has reference to the general aspect or physiognomy of the vegetation and to the plant forms: the floristic composition of the various sclerophyllous—and other physiognomically allied—associations in the various geographical districts is very different; and indeed it is true that, just as the general physiognomy of plant associations is related to climate, so their floristic composition is related to geographical position. Hence, in any cosmopolitan treatment of vegetation, it is necessary to consider the groups of plant communities from the standpoint of the climatic or geographical district in which they occur; and this
1 See Moss, Rankin, and Tansley, " British Woodlands." Botany School (Cambridge, 1910).indeed is consistently done by Schimper. Finally, within any district of constant or fairly constant climatic conditions, it is possible to distinguish plant communities which are related chiefly to edaphic or soil conditions; and the vegetation units of these definite edaphic areas are the plant formations of some writers, and, in part, the "edaphic formations" of Schimper.
When a district like England is divided into edaphic areas, a general classification such as the following may be obtained:
1. Physically and physiologically wet habitats, with the accompanying plant communities of lakes, reed swamps, and marshes. *2. Physically wet but physiologically dry habitats,2 with the accompanying plant communities of fens, moors, and salt marshes.
3. Physically and physiologically dry habitats, with the accompanying plant communities of sand dunes and sandy heaths with little humus in the soil.
4. Habitats of medium wetness, with the accompanying plant communities of woodlands and grasslands. This class may be subdivided as follows:
a. Habitats poor in mineral salts, especially calcium carbonate, often rich in acidic humous compounds, and characterized by oak and birch woods, siliceous pasture, and heaths with much acidic humus in the sandy soil.
b. Habitats rich in mineral salts, especially calcium carbonate, poor in acidic humous compounds, and characterized by ash woods, beech woods, and calcareous pasture.
Ecological Adaptations.—It is now possible to consider the ecological adaptations which the members of plant communties show in a given geographical district such as western Europe, of which England of course forms a part. In the present state of knowledge, however, this can only be done in a very meagre fashion; as the effect of habitat factors on plants is but little understood as yet either by physiologists or ecologists.
Hydrophytes and hemi-hydrophytes (aquatic plants).—Of marine hydrophytes, there are, in this country, only the grass-wracks (Zostera marina and Z. nana) among the higher plants. Even these species are sometimes left stranded by low spring tides, though the mud in which they are rooted remains saturated with sea-water. Although many plants typical of fresh water are able to grow also in brackish water, there are only a few species which appear to be quite confined to the latter habitats in this country. Such species perhaps include Ruppia maritima, R. spiralis, Zannichellia maritima, Z. potycarpa, Potamogeton interrupt us (= P. flabellatus), and Naias marina.,
In freshwater lakes and ponds, especially if the water is stagnant, aquatic plants are abundant. Aquatic vegetation may be conveniently classified as follows:
Aquatic plants with submerged leaves: Chary spp., Naias spp., Potamogeton pectinatus, Ceratophyllum spp., Myriophyllum spp., Hottonia palustris, Utricularia spp.
Aquatic plants with submerged and floating leaves: Glyceria fluitans, Ranunculus peltatus, Nymphaea (Nuphar) • lutea, Callitriche stagnalis, Potamogeton polygonifolius.
Aquatic plants with floating leaves: Lemna spp., Hydrocharis Morsus-ranae, Castalia (Nymphaea) alba.
Aquatic plants with submerged leaves and erect leaves or stems: Sagittaria sagittifolia, Scirpus lacustris, Hippuris vulgaris, Sium lati olium.
Aquatic plants with erect leaves or stems (reed swamp plants) : Equisetum palustre, Phragmites communis, Glyceria aguatica, Carex riparia, Iris Pseudacorus, Rumex Ilydrolapathum, Oenanthe fistulosa, Bidens spp.
Marsh plants: Alopecurus geniculatus, Carex dis!icha, Juncus spp., Caltha palustris, Nasturtium palustre.
In many aquatic plants, the endosperm of the seed is absent or very scanty. The root-system is usually small. Root-hairs are frequently missing. The submerged stems are slender or hollow. Strengthening tissue of all kinds (and sometimes even the phloem) is more or less rudimentary. The stems are frequently characterized by aeration channels, which connect the aerial parts with the parts which are buried in practically airless mud or silt. Submerged leaves are usually filamentous or narrowly ribbon-shaped, thus exposing a large amount of surface to the water, some of the dissolved gases of which they must absorb, and into which they must also excrete certain gases. Stomata are often absent, absorption and excretion of gases in solution being carried on through the epidermal layer. Chloroplastids are frequently present in the epidermal cells, as in some shade plants. Very few aquatic plants are pollinated under water, but this is well-known to occur in species of Zostera and of Naias. In such plants, the pollen grains are sometimes filiform and not spherical in shape. In the case of aquatic plants with aerial flowers, the latter obey
2 As very little experimental work has been done with regard to physiological dryness in physically wet habitats, any classification such as the above must be of a tentative nature.
the ordinary laws of pollination. Heterophylly is rather common among aquatic plants, and is well seen in several aquatic species of Ranunculus, many species of Potamogeton, Sagittaria sagittifolia, Scir pus lacustris, Castalia (Nymphaea) alba, Hippuris vulgaris, Callitriche spp., Sium latifolium.
Insectivorous species occur among aquatic plants; e.g. Utricularia spp., which are locally abundant in peaty waters, are insectivorous.
Xerophytes.—These plants have devices (a) for procuring water, (b) or for storing water, (c) or for limiting transpiration; and these adaptations are obviously related to the physically or physiologically dry habitats in which the plants live. Plants of physically dry habitats, such as deserts and sand dunes, have frequently long tap-roots which doubtless, in some cases, reach down to a subterranean water supply. The same plants have sometimes a superficial root system in addition, and are thus able to utilize immediately the water from rain showers and perhaps also from dew, as Volkens 2 maintains. Root-hairs give an enlarged superficial area to the roots of plants, and thus are related to the pro-curing of water.
The stems of some xerophytes, e.g. Cactaceous and Crassulaceous plants, may be succulent, i.e. they have tissues in which water Is stored. Some deserts, like those of Central America, are specially characterized by succulents; in other deserts, such as the Sahara, succulents are not a prominent feature. Other xerophytes again are spinous. " Switch plants," such as Retama Retam and broom (Cytisus scoparius), have reduced leaves and some assimilating tissue in their stems; and stomata occur in grooves on the stem.
The transpiring surface of xerophytes is frequently reduced. The ordinary leaves may be small, absent, or spinous. In " cushion plants " the leaves are very small, very close together, and the low habit is protective against winds. The latter, of course, greatly increase transpiration. A " cushion plant " (Anabasis aretioides) of the north-western Sahara, frequently shows dead leaves on the exposed side whilst the plant is in full vigour on the sheltered side. The buds and leaves on the exposed side are probably killed by sand blasts. Many xerophytes are hairy or have sunken stomata which may be further protected by partial plugs of wax: the stomata are frequently in grooves: the leaves are frequently rolled—sometimes permanently so, whilst sometimes the leaves roll up only during unfavourable weather. These adaptations tend to lessen the amount of transpiration by protecting the stomata from the movements of the air. In species of Eucalyptus, the leaves are placed edge-wise to the incident rays of light and heat. The coriaceous leaves of " sclerophyllous plants " also, to some extent, are similarly protective. In such leaves, there are a well-marked cuticle, a thick epidermis. a thick hypodermis at least on the upper side of the leaf, well-developed palisade tissue, and a poorly developed system of air-spaces. Such adaptations are well seen in the leaf of the holly (Ilex aquifolium). Warming, however, states that " Ilex aquifolium is undubitably a mesophyte " (1909: 135).
Halophytes, or plants which live in saline soils, have xerophytic adaptations. A considerable proportion of halophytes are succulents, i.e. their leaves and, to some extent, their stems have much water-storing tissue and few intercellular spaces. Some halophytes tend. to lose their succulence when cultivated in a non-saline soil; and some non-halophytes tend to become succulent when cultivated in a salty soil; there is, it need scarcely be stated, little or no evidence that such characters are transmitted. British salt marshes furnish few instances of spiny plants, though such occur occasionally on the inland salt marshes of continental districts. Salsola Kali is British, and a hemi-halophyte at least; and it is rather spiny. Warming states that " the stomata of true, succulent, littoral halophytic herbs, in cases so far investigated, are not sunken " (1909 : 221). It is possible, however, that the absence of sunken stomata, and the occurrence of some other halophytic features, are related merely to the succulent habit and not to halophytism, for succulent species often occur on non-saline soils. Similarly, the small amount of cuticular and of epidermal protection, and of lignification in succulent halophytes may also be related to the same circumstance. Forms of " stone cells " or " stereids " occur in some of the more suffruticose halophytes, as in Arthrocnemum glaucum. The interesting occurrence of certain halophytes and hemi-halophytes on sea-shores and also on mountains is probably to be explained by the past distribution of the species in question. At one time, such plants were probably of more general occurrence: now they have been extirpated in the intermediate localities, chiefly owing to the cultivation of the land in these places by man. In the west of Ireland and in the Faroes, where certain inland and lowland localities are still uncultivated, Plantago maritima and other halophytes occur in quantity and side by side with some " Alpine species," such as Dryas octopetala.
The effect of common salt on the metabolism of plants is not understood. I.esage2 has shown that the height of certain plants is decreased by cultivation in a saline soil, and that the leaves of
Volkens, Die Flora der agyptisch-arabischen Witste (Berlin, 1887).
2 Lesage, " Recherches experimentales sur les modifications des feuilles chez les plantes maritimes," in Rev, gen. de bat. (189o), vol, ii.plants under such conditions become smaller and more succulent. He showed further, that the increase of common salt in the soil is correlated with a reduction in the number and size of the chloroplastids, and therefore in the amount of chlorophyll. On the other hand, some plants did not respond to the action of common salt, whilst others were killed. Warming (1909: 220) quotes Griffon (1898), to the effect that " the assimilatory activity is less in the halophytic form than in the ordinary form of the same species." Schimper had previously maintained that the action of common salt in the cell-sap is detrimental as regards assimilation. Many marine Algae appear to be able to regulate their osmotic capacity to the surrounding medium; and T. G. Hill has shown that the root-hairs of Salicornia possess this property. There has, however, been performed upon halophytes very little physiologically experimental work which commands general acceptance.
Bog Xerophytes live in the peaty soil of fens and moors which are physically wet, but which are said to be physiologically dry. Related to the physiological drought, such plants possess some xerophytic characters; and, related to the physical wetness, the plants possess the aeration channels which characterize many hydrophytes and hemi-hydrophytes. The occurrence of xerophytic characters in plants of this type has given rise to. much difference of opinion. It is sometimes maintained, for example, by Schimper, that their xerophytic characters are related to the physiological dryness of the habitat: this, however, is denied by others who maintain (Clements, 1905: 127) that the xerophytism is due to the persistence of ancestral structures. It is possible, of course, that each explanation is correct in particular cases, as the views are by no means mutually exclusive. With regard to the occurrence of plants, such as Juncus effusus, which possess xerophytic characters and yet live in situations which are not ordinarily of marked physiological dryness, it should be remembered that such habitats are liable to occasional physical drought; and a plant must eventually succumb if it is not adapted to the extreme conditions of its habitat. The xerophytic characters being present, it is not surprising that many marsh plants, like Juncus effusus and Iris pseudacorus, are able to survive in dry situations, such as banks and even garden rockeries.
Tropophytes.—These plants are characterized by being xerophytic during the unfavourable season. For example, deciduous trees shed their leaves in winter: geophytes go through a period of dormancy by means of bulbs, rhizomes, or other underground organs with buds; whilst annuals and ephemerals similarly protect them-selves by means of the seed habit. All such plants agree in reducing transpiration to zero during the unfavourable season, although few or no xerophytic characters may be demonstrable during the period favourable to growth.
Hygrophytes.—Living, as these plants do, under medium conditions as regards soil, moisture and climate, they exhibit no characters which are markedly xerophytic or hydrophytic. Hence, such plants are frequently termed mesophytes. Assimilation goes on during the whole year, except during periods of frost or when the plants are buried by snow. An interesting special case of hygrophytes is seen with regard to plants which live in the shade of forests. Such plants have been termed sciophytes. Their stomata are frequently not limited to the underside of the leaves, but may occur scattered all over the epidermal surface. The epidermal cells may contain chlorophyll. Strengthening tissue is feebly developed. Many sciophytes are herbaceous tropophytes, and are dormant for more than half the year, usually during late summer, autumn and early winter. It may be that this is a hereditary character (cf. " bog xerophytes "), or that the physical drought of summer is unfavourable to shade-loving plants. In this connexion, it is interesting that in the east of England with the lowest summer rainfall of this country, many common sciophytes are absent or rare in the woods, such, for example, as Melica uniflora, Allium ursinum, Lychnis dioica, Oxalis Acetosella, and Asperula odorata. However, the cause of the absence or presence of a given species from a given locality is a department of ecology which has been studied with little or no thoroughness.
Calcicole and Calcifuge Species.—Plants which invariably inhabit calcareous soils are sometimes termed calcicoles; calcifuge species are those which are found rarely or never on such soils. The effect of lime on plants is less understood even than the effect of common salt. Doubtless, the excess of any soluble mineral salt or salts interferes with the osmotic absorption of the roots; and although' calcium carbonate is insoluble in pure water, it is slightly soluble-) in water containing carbon dioxide. In England, the following species are confined or almost confined to calcareous soils: A splenium Ruta-muraria, Melica nutans, Carex digitata, Aceras anthropophora, Ophrys apifera, Thalictrum minus, Helianthemum Chamaecistus, Viola hirta, Linum perenne, Geranium lucidum, Hippocrepis comosa, Potentilla verna, Viburnum Lantana, Galium asperum (= G. sylvestre), Asperula cynanchica, Senecio campestris. The following plants, in England, are calcifuge: Lastraea Oreopteris, Holcus mollis, Carex echinata, Spergula arvensis, Polygala serpyllacea, Cytisus
2 T. G. Hill, " Observations on the Osmotic Properties of the Root-Hairs of certain Salt Marsh Plants," in The New Phytologist (1908), vol. vii.
scoparius, Potentilla procumbens, Callum hercynicum (=G. saxatile), Gnaphalium sylvaticum, Digitalis purpurea. Other plants occur indifferently both on calcareous and on non-calcareous soils.
It is sometimes said that lime acts as a poison on some plants and not on others, and sometimes that it is the physiological dryness of calcareous soils that is the important factor. In relation to the latter theory, it is pointed out that some markedly calcicole species occur on sand dunes; but this may be due to the lime which is frequently present in dune sand as well as to the physical dryness of the soil. Further, no theory of calciolous and calcifugous plants can be regarded as satisfactory which fails to account for the fact that both kinds of plants occur among aquatic as well as among terrestrial plants. Schimper (1903: 102) thinks that in the case of aquatic plants, the difference must depend on the amount of lime in the water, for the physical nature of the sub-stratum is the same in each case. Again, acidic humus does not form in calcareous soils; and hence one does not expect to find plants characteristic of acidic peat or humus on calcareous soils. Some such species are Blechnum boreale, Aira flexuosa, Calluna vulgaris, Vaccinium, Myrtillus, Rubus, Chamaemorus, Empetrum nigrum, Drosera spp. Some, at least, of these species possess mycorhiza in their roots, and are perhaps unable to live in soils where such organisms are absent.
In England, the number of calcicole species is greater than the number of silicolous species. It would therefore be curious if it were proved that lime acts on plants as a poison. It is said that some plants may be calcicoles in one geographical district and not in another. However, until more is known of the exact chemical composition of natural—as contrasted with agricultural—soils, and until more is known of the physiological effects of lime, it is impossible to decide the vexed question of the relation of lime-loving and lime-shunning plants to the presence or absence of calcium carbonate in the soil. From such points of view as this, it is indeed true, as Warming has recently stated, " that ecology is only in its infancy." (C. E. M.)